Method of manufacturing bifocal lenses



July 26; 1932- F. B. wA'rsoN E1' AL 1,868,863

METHOD OF MANUFACTURING BIFOCAL LENSES Original Filed Feb. 26. 1927 F|e.1 I Fm. 2 pms A TT ORNE YS so v sion located nearer the center of the counter- Patented July 26, 1932 l UNITED sTATEs `PA-Tst'rT 'OFFICE FRANK B. WATSON AND EDWARD CUIVER, OF LONDON, ENGLAND, ASSIGNORS TO UNITED 'KINGDOM OPTICAL GOIMIPANY,` LIMITED, OF LONDON, ENGLAND I METHOD OF MANUFACTURING IBIFOGAL LENSES Original Aapplication iiled February 26, 19.27, Serial No. 171,217. Divided and this' application iiled August 2s, 1929. serial' No. 398,911.

Our invention relates to a method of manufacturing fused bifocal lenses. It is the object of our invention to provide a method of fusing bifocal lenses so that the fused inserts may be positioned at varying positions relative to the main lens or countersink notwithstanding the variation in lli form and size of the insert with respect to the countersunk portion of the main lens.

It is a further object to provide a method of manufacturing such lenses so that they will be accurately positioned initially when the fusing starts, and will be automatically maintained in the'accurate position desired during the fusing, so that the resulting lens will be accurately formed.

It is a further object to provide such steps in our method that the resulting lenses will be homogeneous, and mechanically and optically perfect, despite the fact that they are produced in quantities.

This application is a'division of application Serial No. 171,217, filedFebruary 26,-

' Referring to the drawing:

Figure 1 is a plan view of a countersink with the circular depression for receiving the button located near the margin of the countersink;

Figure 2 is asimilar view with the depres- Figure 3 is a view showing the countersink depression and the countersink itself geometrically centered; y

Figure 4 is a plan view of one of the inserts constituting a portion of the button, such as the crown insert;

Figure is a side elevation of a group of flint sections joined together for uniform polishing and finishing on the edges thereof;

Figure 6 isa semi-circular section of one of the flint button sections; p

Figure 7 is a side elevation of a pair of button sections held together by a'clip for fusing;

Figure 8 is one side of one of such sections showing the gray edge thereof on the crown portion;

Figure 9 is an' end elevation of a series of sections constituting the button being held 1n pos1t1on for fusing, the center section being flint and the side sections being crown on the sides of the Hint section of the button;

Figure 10 is a side elevation thereof show- Figure 17 is a side elevation of the cari borundum slab used for fusing the segments to one another, with a clipped assemblage of button segments thereon;

1v,Figure 18 is a plan view of the completed ens.

Referring to the details of our method, we

prepare molded blanks of crown glass 1, pref- -erably of index 1.5232, ground and smoothed in order to fit carborundum slabs 2.

We then grind the counterslnk to the -circular depression 3 in any one of the locations Y shown in Figures 1, 2 and 3 depending upon the position of the insertas desired in the finis ed lens.

In locating these depressions, we keep 1n mind that we desire the finished product to have a single Igeometrical optical center for both distance and reading portions as near as may be possible, although this is not essential because many of our lenses are not so arranged.

One of the fundamentals of our invention is to `provide these .countersink portions with .a polish very much iner and more accurate,

absolutely free from any pitting.

Heretofore, a good felt polish finish has been suicient, but wev find it necessary to use a material consisting of fiber, pitch and `wax' so as to insure the depressions being absolutely free from holes or what is known as curdly surface, thereby eliminating any Y one oblon Hint segment.

We prefer to have the edges of the crown segment unpolished or gray, as indicated at 4a in Figure 8, or Figure 10.

As to the int sections, one edge 5a is then ground plano and finished until itis a constant free from any small pits. It is desirable to have this finished with this uniform surface, but not highly polished so as to avoid any light reflections which would result from high polishing, i.V e. it is given a semipolish where it meets the crown segments.

The flint portion of the segment should be an extra dense flint with an index of 1.64.95V

to 1.652, and of proved expansion qualities. These flint portions of the glass are cut into oblong slabs 5. The sides are smoothed plano and-they may be assembled into groups side to side, as in Figure 5, by means of the Swedish pitch. A good wax maybe used. The top and bottom edges are then ground and reduced to approximately 91/4` millimeters with both sides parallel and smoothed finely, so that there are no pits.

They are then lightly polished with a pitch polish which keeps them free from aberration and which does not form small pits as with the ordinary felt polish.

They are then cleaned with a solvent and dried with a cloth of such a character that it ifloes not leave any lint or threads on the surace.

We join the edges of these segments'by holding them together in a clip 6 and fusing them together. This fused button shown in either Fi res 11, 12, or 15, consisting of a portion o crown glass and a portion of flint glass is located in the depression 3 of the countersink 1 which is mounted on the carborundum base 2. It is held in position by a clip 7, which engages the top of the button and the bottom of the carborundum slab.

In order to position the button in the countersink, we provide angled feelers 8, one portion of which is below the button and the other portion of which is against the edge, which feelers serve to hold the button in position and completely melt into the lens without injuring it throughthe fusing process, and thereby provide means of positioning the lens and compensating for the conditions which occur through fusing, so that the insert will not be displaced. This zinc feeler is later removed by grinding.

We find that, due to holding the buttons in position `by means ofthe spring clips with Lacasse very slight pressure that this has a slight tendency to raise one end or side of the segment from the countersinlr with the ultimate result of `imperfect fusing caused with imperfect contact unless feelers are used.

These feelers, when placed in proper positions in the countersunlr portion of the main lens, hold the flint insert away from the countersink, permitting it to drop evently for perfect fusing. These feelers are sheet zinc and will melt before the fusing temperature is high enough for this flint insert to drop.

The combination as shown in Figures 15 and 16 is then inserted in the furnace, preferably in an electric furnace, and the temperature raised to about 700 degrees centigrade and kept at that temperature for about fifteen minutes. i

The furnace is then allowed to cool about fifty degrees in the nextV thirty minutes` whereupon the current is shut off and th YVlenses-cool in the closed furnace gradually for a period of about fteen hours. y

It will be understood that the crown portions of the buttons are of the same refractive index and expansion as the countersinlr or distance portion. They are also preferably of the same material as the countersink in order that there shall not be the slightest dispersion or variation in the lenses as a finished product because the crown portions must be fused homogeneously. y

The clips used in all fusing operations are made of Monel metal or other similar noncorrosive material.

Care must be taken to prevent any dust between contact surfaces as this will result in air bubbles. A

The segments are now shanked or chipped to circles as large as possible and one side is brightened plano. They are turned then to the other side where the contact curve is ground as desired. y

Great care must be exercised in not having the slightest rounding or aberration at the dividing lines which would render the combination useless for fusing tothe countersink.

In order that the accuracy of the fit between the button and countersink may be understood, it is desirable and essential that the segment must be nearly the same by color contact, which is a. most exacting method of testing curvatures.

In preparing the segments, it is desirable to leave one edge, that is the outer edge, quite rough so that it may be readily distinguished from the contact side which engages with the other portion of the segment.

This refers to the crown portions of the segments. It will be understood that the construction shown in Figures and 9 and Figures 11 and l2, when assembled, are placed in the furnace which is raised to 620 degrees centigrade minutes before being allowed to slowly ton has imparted to it the curvature of the.

.lill

cool.

During the heating, they are mounted on carborundum-slabs which are dusted with powdered mica to prevent the soft glass from sticking.

It is after this time that the resulting butbutton shown in Figure 15. The side on which the mica has vbeen placed shouldybe brightenedplano and fairly Well polished, but no s ecial care is necessary as to the holes or roun ing. y

The other side is then worked to the proper curvature to fit in the countersink.

When the fusing takes place, the button sections drop together' as a unit so as tofit perfectly in the depression; the crown por-- tionsof the button fuse into the crown coun-l tersink and the whole becomes an internal unit there being a visible, Abut not optically objectionable line of division between the flint segment and the crown countersink. Ob.- viously after the fusions, the outer edges of the flint portions may be chipped to conform roughly to the configurations of the desired button.

It will be understood that We desire to comprehend within our invention such modification as may be necessary to adapt it to varyin@ conditions and uses.

laving thus fully described our invention, what we claim as new and desire to secure by Letters Patent, is: s

1. In a process Aof forming a button, joiningiint and crown portions to one another, holding them in engagement with one another, mounting them on a carborundum slabv covered with mica, fusing in a furnace at 620 degrees centigrade for fifteen minutes, allowing to cool in the furnace for fifteen hours, removing fromthe furnace, cutting' the margins of the button to the deslred configuration and finishing the sides thereof.

2. The method of making bifocal lenses which consists in countersinking a crown- 'glass major-lens element, forming a button for the minor-lens element by fusing together edgeto edge a flint-glass segment and a' crown-glass segment of like index with that of the major-lens element, finishing .the button on one side to fit the countersink with optical accuracy, supporting said button on said major element at the outer edge of said cavity by contact of said button segment of like index with said major element, the remainder of said button bein held separated from said major element zinc feelers, mounting said assemblage o elements on a carborundum support dusted with powdered mica or other non-adhesive insulation, heating the whole in a furnace for about fifteen minutes at about 700 degrees centigrade reducing the heat to about 650 degrees for about thirty minutes, and letting the lenses cool and season in the furnace for about fifteen hours.

3. The method of making bifocal lenses which consists in forming a countersink -major lens, forming a button and placing it in the countersink onl the major lens, mounting said'assemblage on a carborundum support dusted -with powdered mica,heating the whole in a furnace for about fifteen minutes at about seven hundred de ees C., reducing the heat to about six hun red and fifty degrees for about thirty minutes, and letting the lens cool and lseason in the furnace for `about fifteen hours.

In testimony whereof we affix our signatures.

. FRANK B.'WATSON. EDWARD CULVER. 

